When I eventually upgrade to 11.04 I will need to install Python 2.6 for development purposes. The 11.04 default is 2.7. Can I just install the python2.6 package alongside the default python package? I don't want anything else to depend on it, I need it for only one purpose. I will be using Distribute and virtualenv to install any further dependencies for Python 2.6.

Done. Now you may want to do pyenv rehash. Also use pyenv versions, since this will show all the versions possible. pyenv local 2.7.6 will set the version of python used by the current directory and below to that version.